During the fabrication of CMOS devices, various processing steps such as plasma deposition and plasma etching can deleteriously influence underlying components of uncompleted CMOS devices. Charged particles, generated during such plasma procedures, can damage the already formed thin gate insulator layers used for the CMOS devices, for example. Protection devices such as protection diodes can be used to guide the unwanted currents generated by the plasma procedures to benign locations in the semiconductor substrate, thus not adversely affecting CMOS device yield or performance.
It is advantageous to form the protection device at an early stage of the CMOS device fabrication sequence so that they are in place during as many of the subsequent plasma processes as possible. Subsequent processing steps such as the formation of heavily doped source/drain regions can counterdope unprotected regions of a protection device such as a diode which, when overlaid with a conductive layer such as metal silicide, can result in unwanted leakage of the protection device, thus reducing or eliminating the ability of the protection device to protect the sensitive components of CMOS devices.
Fringe type protection diode structures include a doped portion formed in regions of a semiconductor substrate surrounded by shallow trench isolation (STI) regions, and are particularly vulnerable to counterdoping phenomena during the subsequent source/drain formation procedures that involve heavy doping. The topography at the interface between the STI, which extends above the substrate surface, and the protection device, formed in the substrate surface, can present problems when attempting to completely cover the protection diode device with a photoresist shape for blocking-out subsequently implanted species used in the formation of heavily doped CMOS source/drain regions, for example. The photoresist shape can become notched, or thinned at this topographical interface allowing unwanted species to enter the peripheral regions of the protection diode device near the interface. If the undesirably implanted species is of opposite conductivity (i.e., counterdoping occurs) a subsequent metal silicide contact layer formed to overlay both the main doped portion of the protective diode, and the oppositely doped component at the periphery, provides a direct path through the protection diode device to regions of the semiconductor substrate, negating the purpose of the protection diode.
The present invention advantageously provides a structure and procedure whereby the protection diode structure, even if undesirably counterdoped during subsequent implant procedures, still remains effective after formation of an overlying metal silicide layer, and therefore still functions to direct any unwanted charge generated during subsequent plasma procedures, to non-CMOS regions of the semiconductor substrate. This is accomplished via masking procedures obtained using no additional process or masking steps. References such as Voldman et al, in U.S. Pat. No. 5,629,544, Chang et al., in U.S. Pat. No. 6,294,448, Wang et al., in U.S. Pat. No. 6,291,281 and Jang, in U.S. Pat. No. 6,093,593, describe methods of forming protective structures, or describe materials used to avoid leakage paths through protection diode devices, but fail to provide a structure that effectively prevents an overlying conductive layer, such as metal silicide, from shorting the counterdoped region of the protection diode structure to an underlying well region. The present invention addresses this shortcoming of conventional technology.